How to do (external acces for specific task)

I run a task which is shared with colleagues.
This task is specific to a project but it must be carried out by an outside person.
How to give it access without seeing the other task?
Because giving public access gives him access to the whole of the project.
(this can be useful in case of subcontracting)
Thank you to you

even playing with the rights of users can not do it !!!